1. Field of the Invention
The present invention relates to a data carrier, for example a document of value, with at least one halftone image represented by structural elements, each structural element having a basic geometry and a size whereby the size of the structural element represents a gray level of the halftone image, and to a method for producing the same. The invention further relates to a data carrier, for example a document of value, with at least one picture element represented by one or more structural elements.
2. Related Art
A special problem with documents, such as documents of value, is the protection from forgery, in particular by copying or scanning an authentic document to produce a falsified document. For example, EP 0 710 574 A2 relates to a security document with a drawing whereby a moire pattern arises in the corresponding drawing on a copy of the security document. For this purpose a whole-area screen structure with parallel lines is provided. The drawing is done in the form of a half-tone image, the thicknesses of the lines being varied in a contact screen structure as described in EP 0 085 066 B1. Further, the distance between the lines is varied over the entire halftone image in accordance with a modulation function. That is, the number of lines per unit length varies over the total surface of the drawing. Modification of such a line density leads to copy protection against color copiers or scanners since the superimposition of the screen structure at least in a predetermined area with the scanning screen of the copier or scanner produces a very striking moire pattern. Since this moire pattern can be seen only on the copy, not on the original, the copy is easily identifiable as a forgery.
Although there is a moire effect at least in predetermined areas through the variation of line density even with different scanning screens of the scanner, the variation of line density over the total surface of the drawing has an extremely adverse effect on the optical appearance of the half-tone image. The alternation between high and low density or number of lines per unit area causes the picture to seem restless and inhomogeneous and the screen pattern to dominate the halftone image rather than vice versa, so that the picture is not very appealing esthetically.
The present invention is therefore based on the problem of providing a document with a-moire-producing structure, in particular on a halftone image, whereby large-surface moire structures are produced upon copying of the document for detecting forgeries, the moire-producing structure simultaneously fitting homogeneously into the halftone image and receding as a background structure relative to the halftone image itself.
The invention is based on the idea of dividing the total surface of a picture in which moire structures are to be produced upon copying into a plurality of areas. Each area has associated therewith a number of structural elements for producing the gray levels present in this area. The number of structural elements is selected in at least two contiguous areas so as to be different in the two contiguous areas. This different number results necessarily in an offset of the structural elements relative to the structural element of the adjacent area. Thus, the halftone image applied to the data carrier is divided into areas which have different screen frequencies. Upon an attempt to copy this halftone image or read it into a data processing system with a scanner, the scanning frequency of the scanner or copier is superimposed with the applied, different screen frequencies of the half-tone image. This superimposition leads to disturbances in the reproduction of the halftone image, this disturbance being apparent in particular in the production of a large-surface moirxc3xa9 pattern.
The variation of the number of structural elements in the areas of the halftone image produces a different screen frequency for each area, thereby ensuring that a moirxc3xa9 pattern arises even when the scanning screen, i.e. the scanning or copying frequency, is varied. This then appears in the areas of the total surface for which the scanning and screen frequencies are coordinated with each other such that a moirxc3xa9 pattern can arise.
According to the invention the image thus has predetermined areas each having a predetermined number of structural elements, the number of structural elements per unit area of an area being different between at least two contiguous areas and/or the structural elements being offset from each other in at least two of the areas. This has the advantage that disturbances such as moirxc3xa9 patterns arise even with different scanning screens, for example of a copier or scanner, without inhomogeneities arising in the total surface screen, in particular in the halftone image.
In a preferred embodiment the structural elements of an area of the halftone image have a uniform basic geometry, it being particularly preferable for the structural elements of all areas to have a uniform basic geometry. The structural elements are preferably executed as lines, a predetermined thickness of a line representing a predetermined gray tone separately for each area. This makes it possible to ensure a homogeneous brightness level over the total gray-level image despite the gray-level image being divided into a plurality of areas with different numbers of structural elements. If for example n structural elements are present in a first area and n+10 structural elements in the adjacent area, the second area would appear optically darker than the first area solely due to the increased number of structural elements. This difficulty is avoided if a given width of the line corresponds to a given gray tone within one area, while a different, for example smaller, width of the line is provided for the same given gray tone within a second area having a higher number of lines in this case.
The inventive representation of halftone images by areas with different numbers of structural elements thus achieves the advantage that the halftone image has different screen frequencies which are superimposed with the scanning frequency of a scanner or copier used for scanning the document, and the different screen frequencies produced by the varied number of structural elements per area offer the possibility of superimposing the scanning frequencies with a plurality of screen frequencies, thereby clearly increasing the probability of a moirxc3xa9 pattern forming. Simultaneously the effect of individual areas darkening due to the increased number of structural elements is avoided since the predetermined size of the structural elements corresponds to a predetermined gray tone within an area, but the predetermined sizes of the structural elements in the different areas can represent different gray tones depending on the number of structural elements in each area.